1. Field of the Invention
The present invention relates to valves for controlling the flow of a fluid, in general, and to such apparatus for controlling the flow of a fluid in constant flow pumping apparatus, in particular.
2. Description of the Prior Art
In the operation of the present-day coating apparatus where coatings of critical thickness are desired to be deposited on a substrate such as a moving web for use in photographic film, it is necessary to precisely control the flow rate of liquid coating material from a liquid storage reservoir to a coating applicator such as an extrusion coater, which is one device that applies coating material to such a web. An extremely important factor for the control of coating thickness and variations in coating thickness is the control of the flow rate of the material of which said coating is formed, a said coating is being deposited on a substrate.
One piece of apparatus for use is controlling the flow of a coating fluid onto a substrate is the constant flow pump described in our copending U.S. patent application Ser. No. 108,700, filed on Dec. 31, 1979 (now U.S. Pat. No. 4,321,014) and assigned to the assignee of the present invention. In this pumping apparatus, it is necessary to continually interrupt the flow of coating material through portions of said pumping apparatus with valving, for proper pump operation.
Conventional valves utilize valve members that change the volume of the fluid they are controlling as a movable valve member moves into and/or out of its valve seat. Portions of the valve stem, for example that were outside of the valve housing when the valve was opened are moved inside the valve housing as the valve is moved to its valve seat, thereby displacing the controlled fluid and increasing its effective volume. If such conventional valve were utilized in the pumping apparatus of the above-cited patent application, unwanted changes in the flow rate of the material pumped by said pumping apparatus would result. In addition, if the flow rate of coating materials for use in photographic film, for example, were interrupted by such conventional valves, shearing of the coating materials would occur as said materials are compressed between the valve and its valve seat, which would detrimentally change the physical properties of the photographic coating materials so compressed.
It is an object of the present invention to provide a valve for controlling the flow of a fluid that will not change the volume of the fluid that it controls when operated between its opened and closed positions and that will minimize the amount of shearing of said fluid when said valve is moved into its valve seat to interrupt fluid flow.